Centos下安装nc命令工具及问题解答

netcat(nc)是一个简单而有用的工具，被誉为网络安全界的“瑞士均道”，不仅可以通过使用TCP或UDP协议的网络连接读写数据，同时还是一个功能强大的网络调试和探测工具，能够建立你需要的几乎所有类型的网络连接。在Linux终端窗口可以直接使用yum工具进行安装：

[root@master01 spark]# yum install nc.x86_64

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安装完毕后，在终端模式下运行nc -help查看命令是否正常安装

[hadoop@master01 spark]$ nc -help
usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
[-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]
[-x proxy_address[:port]] [hostname] [port[s]]
Command Summary:
-4              Use IPv4
-6              Use IPv6
-D              Enable the debug socket option
-d              Detach from stdin
-h              This help text
-i secs         Delay interval for lines sent, ports scanned

fc2d
-k              Keep inbound sockets open for multiple connects
-l              Listen mode, for inbound connects
-n              Suppress name/port resolutions
-p port         Specify local port for remote connects
-r              Randomize remote ports
-S              Enable the TCP MD5 signature option
-s addr         Local source address
-T ToS          Set IP Type of Service
-C              Send CRLF as line-ending
-t              Answer TELNET negotiation
-U              Use UNIX domain socket
-u              UDP mode
-v              Verbose
-w secs         Timeout for connects and final net reads
-X proto        Proxy protocol: "4", "5" (SOCKS) or "connect"
-x addr[:port]  Specify proxy address and port
-z              Zero-I/O mode [used for scanning]
Port numbers can be individual or ranges: lo-hi [inclusive]

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OK，安装正常。下面我们就可以使用nc -l 9999 来监听端口，并发送数据；

[hadoop@master01 spark]$ nc -l 9999
Hello world!
How are you?
Goodbye!

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在我的CentOS 6.4(64-bit) 环境中执行 nc -l 9999 监听命令遇到protocol not available。

[hadoop@master01 spark]$ nc -l 9999
nc: Protocol not available

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先先后后把nc卸载再安装了好几次，还是没有解决问题；最终在https://access.redhat.com/solutions/1753753 中找到了解决方法和问题原因。 
解决方法如下：

# Removes the old package
yum erase nc

# Manually downloads the working package from the Official Repository
wget http://vault.centos.org/6.6/os/x86_64/Packages/nc-1.84-22.el6.x86_64.rpm 
# Installs the package
rpm -iUv nc-1.84-22.el6.x86_64.rpm

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另：本系统为64位操作系统，若读者使用的是i386 (32-bit)，可以使用如下链接：

wget http://vault.centos.org/6.6/os/i386/Packages/nc-1.84-22.el6.i686.rpm[/code]

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